1. Field of the Invention
Exemplary embodiments of the present patent application relate to an image forming apparatus that incorporates a sheet feeding unit in which an uppermost sheet placed on top of a sheet stack is attracted to the surface of a dielectric belt by the action of an electric field generated by electric potential patterns formed on the surface of the dielectric belt and fed in a sheet feeding direction as the dielectric belt rotates.
2. Discussion of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a sheet of recording media according to image data. Thus, for example, a sheet feeding unit feeds a plurality of sheets one by one toward an image forming device. The image forming device forms an image on a sheet fed from the sheet feeding device.
The sheet feeding device incorporated in such related-art electrophotographic or inkjet image forming apparatuses often use a friction feed method by including a friction member to separate an uppermost sheet from other sheets of the sheet stack loaded in a sheet cassette. Specifically, the friction member, made of rubber having a high friction coefficient, pressingly contacts the uppermost sheet to separate the uppermost sheet from other sheets and conveys it as appropriate. One problem with such an arrangement is that the high friction coefficient of the friction member, which is necessary to feed the sheets to the image forming device in a stable manner, may deteriorate over time or according to environmental conditions, degrading feeding performance of the sheet feeding unit.
Further, when the image forming apparatus is used as a printer, it handles various types of recording media, such as plain paper, coated paper, and label paper. With recording media having a substantially small friction coefficient, sheets providing friction that varies depending on temperature, or sheets absorbing moisture and adhering to each other, the friction member of the sheet supplier may not separate the uppermost sheet from other sheets properly.
Further still, with recording media such as adhesive labels, the surface portion of the sheet can be easily separated from the underlying base layer of the sheet by the frictional force exerted between the pickup member and the recording medium, hindering reliable pick-up of the recording medium by the friction feeding method.
To address the above-described drawbacks, the image forming apparatus can employ an electrostatic sheet feed method in which recording media are electrically attracted to the surface of a dielectric belt by the action of an electric field generated by electric potential patterns formed on the surface of the dielectric belt and separated from a stack of recording media one by one as the dielectric belt rotates. As examples of an electrophotographic image forming apparatus that employs such an electrostatic sheet feed method, Japanese Patent Application Publication No. 05-139548 (JP-H05-139548-A) and Japanese Patent Application Publication No. 2003-237958 (JP-2003-237958-A1) have been proposed.
However, there is a drawback to the electrophotographic method. In the electrophotographic sheet feed method, if the uppermost sheet is picked up from the sheet stack immediately upon contact with the dielectric belt, several subsequent upper sheets can also be picked up together with the uppermost sheet by the dielectric belt. Therefore, to avoid this problem, the dielectric belt remains in contact with the sheet stack for a predetermined period of time from the moment the dielectric belt contacts the sheet stack before separating from the sheet stack. However, it is now known that subsequent sheets of the sheet stack can be still picked up together with the uppermost sheet even after the predetermined period of time has elapsed.